minor #5554 DX: use tmp file in sys_temp_dir for integration tests (keradus)

This PR was squashed before being merged into the 2.18 branch.

Discussion
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DX: use tmp file in sys_temp_dir for integration tests

Commits
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860747f13 DX: use tmp file in sys_temp_dir for integration tests

tests/IntegrationTest.php

@@ -41,7 +41,7 @@ final class IntegrationTest extends AbstractIntegrationTestCase
      */
     protected static function getTempFile()
     {
-        return self::getFixturesDir().\DIRECTORY_SEPARATOR.'.tmp.php';
+        return tempnam(sys_get_temp_dir(), 'PHP-CS-Fixer');
     }
 
     /**